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FS-1996-08-08-LaRC
August 1996

Satellites - A Global View of Earth

Global environmental change is one of the most pressing
international concerns of the 21st Century. These changes may
include the effects of global warming, stratospheric ozone
depletion, and large-scale changes in land cover due to human
activities such as biomass burning.

In the 1960s, the desire to monitor the Earth's environment was
linked with new technology that enabled us to observe our planet
from space - giving humans our first global view of Earth.

What are satellites?

Before 1957, the word satellite meant one thing -- a small body
that revolved around a larger astronomical body. Today we call
these "natural satellites." In 1957, the Soviets launched the first
"artificial satellite," Sputnik 1. Today there are hundreds of
artificial satellites in orbit around the Earth. These satellites
are used for many purposes, such as communications, weather
forecasting, and navigation, as well as observing the Earth.

Satellites used to observe the Earth carry a variety of
instruments to study the land, ocean, air and life, as well as
their interactions. Scientists at NASA's Langley Research Center
use several satellites now in orbit to measure some very specific
atmospheric phenomena, such as the amount of ozone in the
atmosphere, and to help them distinguish between environmental
changes caused by humans and those that occur naturally.

Data gathering and interpretation

The technology used on a satellite varies, depending on its
mission. Computers aboard a satellite can receive, store and
transmit information in the form of radio signals sent to and from
stations on Earth. For Earth scientists, the receipt of the data
begins a long process of determining what the data means. By
incorporating the data into computer models (which use mathematical
formulas called algorithms) researchers can simulate, or model,
Earth's processes -- how the atmosphere, oceans and land surfaces
interact as a system. Scientists hope that incorporating global
satellite data into their computer models will help them better
understand the interactive roles of Earth's systems, and help them
predict how the Earth's environment will change over time.

It was not until scientists made satellite measurement that
they realized the true extent of global biomass burning. Biomass
burning proved to be 100 times greater than was originally thought.
The above annual data shows the number of fires which burned in
Africa during 1995. Individual fires areas are shown as black
dots.

Some Ongoing NASA Satellite Missions

NASA Langley researchers use many satellites to study
atmospheric variables and their effects

MAPS

The Measurement of Air Pollution from
Satellites (MAPS) instrument produced the first global measurements
of atmospheric carbon monoxide (CO) in 1981 when it flew aboard the
Space Shuttle Columbia (STS-2). MAPS' most important finding was
that air pollution is a worldwide phenomenon, not just a problem in
industrialized countries. In 1981, and in subsequent shuttle
flights in October 1984 and 1994, MAPS measured high values of CO
pollution in the tropics caused by seasonal biomass burning. In
1997, MAPS will be mounted to the Russian space station Mir to
monitor global CO levels during a year of seasonal changes.

ERBE

The Earth Radiation Budget
Experiment (ERBE) is made up of three satellites
launched in the mid-1980s. Since then, ERBE has been the primary
source of global data for studying the heating and cooling of the
atmosphere. This data may tell us the extent to which global
warming is occurring. ERBE technology also measures the effects of
clouds on the exchange of energy between the sun, Earth and space.
The ERBE sensors measure energy from the sun in various
wavelengths: reflected shortwave solar radiation (light that does
not reach the Earth but is reflected off clouds) and longwave
emitted energy (the heat that is emitted into space by the Earth).
By analyzing long-term measurements of these energy components,
scientists can study the Earth's climate. ERBE has provided the
most accurate data ever obtained on short- and longwave radiant
energy, helping us better understand how clouds reflect and absorb
sunlight, and the heat emitted by the Earth into space. NASA
scientists have used this data to make important contributions to
climate prediction by improving how clouds are represented in
atmospheric models.

Earth's energy budget: The exchange of energy between the
sun,
Earth, clouds and space. Numbers are percentages.

Diagram of the solar occultation technique

CERES

The Clouds and the Earth's Radiant
Energy System (CERES) instrument is a follow-on to ERBE. CERES will be
able to better identify cloud properties as well as help scientists
better understand the Earth's energy budget. CERES will be launched
in late 1997 aboard the Tropical Rainfall
Measuring Mission (TRMM) spacecraft, as part
of NASA's Mission To Planet Earth Program.

SAGE I and II

The Stratospheric Aerosol and Gas
Experiment I (SAGE I) measured ozone, particles in
the upper atmosphere (aerosols) and nitrogen dioxide from 1979 to
1981. Using a process called solar occultation, sensors on SAGE I
measured sunlight coming through the atmosphere to determine how
much sunlight was absorbed. The amount of absorption indicates the
amount of various sunlight absorbing gases, like ozone, or
aerosols, that are present. Solar occultation occurs as the
satellite experiences sunrises and sunsets, when the light is not
too bright to obscure readings. SAGE I produced the first global
atmospheric data of this type.

SAGE II began operation in 1984 with the launch of the
Earth Radiation Budget Satellite. SAGE II, which is still operating, provides global
measurements of the vertical structure of ozone, nitrogen dioxide,
water vapor and stratospheric aerosols. The SAGE II data helped
scientists understand the causes and effects of the Antarctic ozone
hole, and has made invaluable contributions to understanding the
decline of stratospheric ozone over the Earth's mid-latitudes.

HALOE

The Halogen Occultation Experiment (HALOE), launched in 1991 aboard the Upper
Atmosphere Research Satellite (UARS),
measures ozone and other atmospheric gases. Like SAGE I and II,
HALOE uses the solar occultation technique; however, it measures
visible infrared light and uses a filter which separates the gases
according to their individual light "signatures." Analysis of the
HALOE data proved conclusively that the Antarctic ozone hole was
caused by human-produced chlorofluorocarbons (CFCs).

Conclusion

The adage, "a picture is worth a thousand words," holds
especially true for satellite data - a global satellite "picture"
can help scientists "see" the whole Earth and better understand its
many interdependent systems. NASA will continue to study the Earth
from space, and improve our satellite remote sensing abilities,
through its ongoing Mission to Planet Earth Program.